1. Field of the Invention
This invention relates to a vibration control apparatus, and in particular to a microgravity vibration control apparatus.
2. Discussion of the Prior Art
At low frequencies ( less than 0.01 Hertz) space platforms such as the shuttle and the International Space Station (ISS) provide a unique, near ideal free-fall environment, which can be used to conduct material science, fluid physics and crystal growth experiments. Departure from ideal free fall due to atmospheric drag, rotational effects and gravity gradient are of the order of a micro-g (10xe2x88x926 g). However, above 0.01 Hz spacecraft vibrations are such that acceleration levels typically exceed 10xe2x88x923 g. Experiments conducted on the space shuttle and on MIR have shown that these vibration levels can significantly affect results. Vibrations, which are sometimes referred to as g-jitter, are driven by on-board activities such as attitude control systems, thermal control systems, air conditioning systems, power generation systems, crew activity and the operation of the spacecraft resulting in vibration environments characterized by milli-g (10xe2x88x923 g) acceleration levels. On the space shuttle, vibration levels in the frequency band 0.01 Hz to 100 Hz are in the range of 10xe2x88x923 g Root Mean Square (RMS), with peaks typically exceeding several milli-g. These are sufficient to cause significant disturbances to experiments that have fluid phases, which includes many material science experiments. The acceleration environment of the International Space Station will likewise not be as clean as originally hoped for, and the ISS will not meet the current vibratory requirements without the use of vibration isolation apparatuses of the type described herein.
In order to isolate fluid science experiments from spacecraft vibrations, the Canadian Space Agency (CSA) developed a so-called Microgravity Vibration Isolation Mount (MIM), which constitutes a first generation of the present invention. The MIM was operational for more than 3000 hours on the Mir space station between May 1996 and January 1998. A second generation MIM was flown on space shuttle mission STS-85 in August 1997.
The MIM includes two major components, namely a stator which is fixed to 10xe2x88x923 the spacecraft and a flotor on which is mounted an experiment to be isolated. Positions sensing devices track the position and orientation of the flotor with respect to the stator, and accelerometers monitor stator and flotor accelerations. The position sensing devices and accelerometers are used in an active control loop including magnetic actuators for moving the flotor relative to the stator to compensate for even extremely small vibrations of the stator.
There is a large volume of patent literature relating to vibration isolation and damping systems. Examples of such literature include U.S. Pat. No. 2,788,457 (Griest); U.S. Pat. No. 3,088,062 (Hudimac); U.S. Pat. No. 4,088,042 (Desjardins); U.S. Pat. No. 4,314,623 (Kurokawa); U.S. Pat. No. 4,432,441 (Kurokawa); U.S. Pat. No. 4,585,282 (Bosley); U.S. Pat. No. 4,595,166 (Kurokawa); U.S. Pat. No. 4,874,998 (Hollis Jr.); U.S. Pat. No. 4,710,656 (Studer); U.S. Pat. No. 4,724,923 (Waterman); U.S. Pat. No. 4,848,525 (Jacot et al); U.S. Pat. No 4,874,998 (Hollis Jr.); U.S. Pat. No. 4,929,874 (Mizuno); U.S. Pat. No. 4,947,067 (Habermann et al); U.S. Pat. No. 5,022,628 (Johnson et al); U.S. Pat. No. 5,168,183 (Whitehead); U.S. Pat. No. 5,236,186 (Weltin et al); U.S. Pat. No. 5,285,995 (Gonzalez et al); U.S. Pat. No. 5,368,271 (Kiunke et al); U.S. Pat. No. 5,385,217 (Watanabe et al); U.S. Pat. No. 5,392,881 (Cho et al); U.S. Pat. No. 5,400,196 (Moser et al); U.S. Pat. No. 5,427,347 (Swanson et al); U.S. Pat. No. 5,427,362 (Schilling et al); U.S. Pat. No. 5,445,249 (Aida et al); U.S. Pat. No. 5,446,519 (Makinouchi et al); U.S. Pat. No. 5,483,398 (Boutaghou); U.S. Pat. No. 5,542,506 (McMichael et al); U.S. Pat. No. 5,584,367 (Berdut); U.S. Pat. No. 5,609,230 (Swinbanks); U.S. Pat. No. 5,638,303 (Edberg et al); U.S. Pat. No. 5,645,260 (Falangas); U.S. Pat. No. 5,718,418 (Gugsch); U.S. Pat. No. 5,744,924 (Lee); U.S. Pat. No. 5,765,800 (Watanabe et al); U.S. Pat. No. 5,844,664 (Van Kimmenade et al); U.S. Pat. No. 5,876,012 (Haga et al); U.S. Pat. No. 5,925,956 (Ohzeki); U.S. Pat. No. 6,031,812 (Liou), and WO 99/17034 (Nusse et al) and WO 00/20775 (Ivers et al).
Some fluid phase experiments require controlled and induced vibration of the experiment, with no reaction back to the space vehicle. While a system of the type described above, including a stator and flotor, provides vibration damping, such a system cannot be used to effect such controlled and induced vibration.
The object of the present invention is to meet the need defined above by providing a vibration control apparatus which can effect controlled and induced vibration of an experiment with no disturbance to the space station. Coincidentally, the apparatus of the present invention is inherently more efficient at damping vibration than a two-stage system.
Accordingly, the invention provides a vibration control apparatus comprising:
(a) stator means for mounting on a fixed surface;
(b) lower flotor means normally spaced apart from said stator means in nesting relationship thereto;
(c) an upper flotor means normally spaced apart from said lower flotor means in nesting relationship thereto;
(d) work platform means on said upper flotor means;
(e) position sensing means associated with said stator means, lower flotor means and upper flotor means for determining the position and orientation of said lower flotor means and said upper flotor means relative to said stator means;
(f) accelerometer means associated with said stator means, lower flotor means and upper flotor means for determining acceleration of said lower flotor means and upper flotor means with respect to inertial space; and
(g) vertical and horizontal magnetic force actuator means associated with said stator means, lower flotor means and upper flotor means for imparting motion to said lower flotor means and to said upper flotor means to compensate for vibration of said stator means, whereby vibration of said work platform is minimized.